Abstract

Nordic plant breeders have selected superior genotypes and released new spring wheat cultivars throughout this century. However, the extent of phenotypic diversity that exists in this improved germplasm has not been accurately investigated. This study phenotypically assessed 75 selected cultivars released to farmers in Denmark, Finland, Norway and Sweden within the last 10 decades. Grouping of cultivars was not related to their geographical origin or decade of release. The respective within‐cluster variances were always larger than the among‐cluster variances. The average phenotypic diversity for Nordic spring wheat germplasm was 0.22, which was not surprising because some newer cultivars were derived from older cultivars. There was a significant influence of geographical origin on days to heading, straw, grain and biomass weight, and kernel number. Likewise, the country × decade of release interaction was significant for all characteristics except plant height. These findings suggest that irrespective of location, Nordic breeders have reduced plant height in recently released cultivars at a rate of —0.5 cm year−1, thereby reducing lodging in this germplasm. There was no significant change in biomass, straw and kernel weight between old and new cultivars. Nonetheless, relative genetic gains in this germplasm during this century were significant for agronomic characteristics, such as days to heading (8%), plant height (36%), grain yield (20%), harvest index (19%), and number of kernels per unit area (18%). On average, the absolute genetic gain for grain yield was about 18.5 kg ha−1 year−1 Negative changes in days to heading (at a rate of —0.06 year−1) and plant height, and positive changes in harvest index (0.06% year−1), and kernels m‐2 (45 year−1) were associated with gains observed in grain yield, i.e. an early flowering plant with short straw, but many fertile tillers, had high grain yield. This sustained genetic gain could result from the accumulation of favourable alleles and intergenomic interactions between homologous loci during systematic plant breeding for grain yield.